This progress report summarize's the work done on 'Improved Design concepts for Millimeter Wave Power Sources,' covering the period from March 15, 1992 to March 14, 1993. In the past year, we have designed a phase-locked, harmonic, inverted gyro-twistron, known as the phigtron. The phigtron combines a subharmonic gyro-TWT amplifier input section with a gyroklystron type output cavity. The phigtron is expected to reach much higher phase-locking gain and wider bandwidth than the two cavity phase-locked gyroklystron oscillator. The efficient and stable of operation of this phase-locked harmonic gyrotron will be obtained through the implementation of mode selective interaction circuits. The proof-of-principle cold test results of such circuitry have recently been obtained indicating that the technical realization of this research concept is feasible. The construction of the phigtron hot test laboratory facility is now actively proceeding.